1. Field of the Invention
The present invention relates to membrane-electrode assemblies, which are utilized for solid polymer electrolyte fuel cells, which may provide the fuel cells with higher power-generating performance due to superior proton conductivity, and which include solid polymer electrolyte membranes having superior toughness and hot-water resistance.
2. Related Art
The solid polymer electrolyte membranes of the membrane-electrode assemblies for fuel cells are generally classified into inorganic and organic types.
The inorganic type is typically exemplified by those containing uranyl phosphate hydrate; however, proton-conductive materials formed of such inorganic compounds typically exhibit poor workability; that is, when electrode layers are being connected, the adhesiveness is often insufficient at the interfaces between the electrode layers and the proton-conductive materials, thus resulting in lower power generating performance due to insufficient proton conductivity at the interfaces.
On the other hand, examples of the organic type include so-called cationic exchange polymers, sulfonated vinyl polymers such as polystyrene sulfonic acid, perfluoroalkyl sulfonic acid polymers such as Nafion (product name, by DuPont), perfluoroalkyl carboxylic acid polymers, and organic polymers of such heat resistant polymers as polybenzimidazoles and polyetheretherketone having sulfonic or phosphoric group introduced therein (see Non-patent Documents 1 to 3 described below).
These organic polymers are typically utilized in the form of a film of a solid polymer electrolyte membrane. In contrast to inorganic materials, the organic polymers may be advantageously processed such that a conductive membrane can be connected onto an electrode because organic polymers are soluble in solvents and are thermoplastic.
Furthermore, Patent Document 1 proposes a solid polymer electrolyte membrane formed of a rigid sulfonated polyphenylene, which is based on a polymer produced by polymerizing an aromatic compound having a phenylene chain and introducing a sulfonic acid group through a reaction with a sulfonating agent.    Non-patent Document 1: Polymer Preprints, Japan, Vol. 42, No. 7, pp. 2490-2492(1993)    Non-patent Document 2: Polymer Preprints, Japan, Vol. 43, No. 3, pp. 736 (1994)    Non-patent Document 3: Polymer Preprints, Japan, Vol. 42, No. 3, pp. 730 (1993)    Patent Document 1: U.S. Pat. No. 5,403,675
However, most of these organic polymers suffer from such problems as insufficient proton conductivity, insufficient durability, decrease of proton conductivity at higher temperatures, e.g., no less than 100 degrees C., significant decrease of mechanical properties, e.g., elastic modulus in particular, significant fluctuation of proton conductivity under various humidity conditions, unsatisfactory adhesion with electrodes, and decrease of strength or disintegration of assemblies induced from excessive swelling due to hydroscopic polymer structures. As such, there exist a variety of problems in the organic polymers, described in the Non-patent Documents for example, for applying to solid polymer electrolyte membranes.
In addition, the organic polymers described in Patent Document 1 described above may provide higher proton conductivity when the additional amount of sulfonic acid group is increased; however, there simultaneously arise problems in that mechanical properties of sulfonated polymers, for example, breaking elongation, folding resistance and toughness, and also hot water resistance, are remarkably impaired.
Accordingly, an object of the present invention is to provide membrane-electrode assemblies (hereinafter referred to sometimes as “MEA”) for solid polymer electrolyte fuel cells that may display superior proton-conductive power generating performance and also exhibit excellent toughness and hot water resistance.